Aga not heating water

My Aga no longer heats the hot water. The flow from the aga to the primatic cylinder gets reasonably hot but the return no longer gets even lukewarm. I assume that the flow pipe is caked with limescale and needs descaling. The air bubble in the cylinder i assume is intact as there is no cross contamination. Is there any way of descaling the pipe without disconnecting the pipes.? What is the best product to use for descaling the pipes. The primatic has had a coil fitted so water can still be heated indirectly by central heating boiler.